1. Field of the Invention
The present invention relates to a method for manufacturing a granular silicon crystal and, in particular, a method for manufacturing a granular silicon crystal suitable for obtaining a granular silicon crystal for use in a photoelectric conversion apparatus.
2. Description of Related Art
A solar battery using a crystal silicon wafer and having high photoelectric conversion efficiency (hereinafter referred to as “conversion efficiency” in some cases) has come into use. This crystal silicon wafer is attained by cutting down a large single crystal or polycrystalline silicon ingot that has excellent crystallinity and less impurities, and has no offset in its distribution. However, a large single crystal or a polycrystalline silicon ingot takes long to manufacture, so that its productivity is poor and hence a wafer to be cut down from such an ingot is expensive. For this reason, there is a strong desire for a next generation solar battery that requires neither a large single crystal nor a polycrystalline silicon ingot and that is low cost in terms of photoelectric conversion efficiency.
A photoelectric conversion apparatus using a granular silicon crystal as a component of photoelectric conversion means is occupying the attention as a promising photoelectric conversion apparatus in the future commercial scene.
As a raw material for manufacturing a granular silicon crystal, there are presently used silicon minute particles resulting from grinding of a single crystal silicon material, and high-purity silicon subjected to vapor-phase synthesis by fluid bed method. The following methods are used to manufacture a granular silicon crystal. That is, the above raw material is classified by size or weight, and then melted in a container by infrared irradiation or high frequency induction heating. This melt is then solidified while allowing it to freely fall as droplets (granular melt) (see for example the pamphlet of International Publication No. WO99/22048). Alternatively, this melt is made in a spherical shape by high-frequency plasma heat melting (see for example the specification of U.S. Pat. No. 4,188,177).
However, the above conventional methods for manufacturing a granular silicon crystal suffer from the problem of poor productivity because it is difficult to homogenize the weight of minute particles of silicon as raw material. Since variations in the minute particles of silicon as raw material is reflected to the size of a granular silicon crystal to be manufactured therefrom, a raw material consisting of minute particles of uniform weight is required. For silicon, it is however difficult to obtain efficiently minute particles as raw material so as to have a particle size of 100 μm to 1000 μm and a small dispersion of particle size by means of grinding and classification.
Further, when obtaining minute particles of silicon by grinding, there may arise contamination from a ground media and hence contamination of impurities is unavoidable.
Additionally, a high frequency plasma heat melting apparatus for obtaining a granular silicon crystal calls for an extremely large power source or the like, and the apparatus cost is high and working power is large, resulting in high production cost.
In the method for manufacturing a granular silicon crystal in which silicon melt is solidified while allowing it to freely fall as droplets (granular melt), silicon melt is discharged and fallen from a nozzle part of a crucible containing the silicon melt. At this time, silicon in the silicon melt is reacted with carbon or nitrogen that is the material of the nozzle part composed of silicon carbide or silicon nitride, which can cause enlargement of a nozzle hole of the nozzle part. In this case, the sizes of droplets of the silicon melt vary in a short period (i.e., if used one or two times), failing to yield granular silicon crystals of uniform particle size at high productivity. Moreover, the nozzle hole of the nozzle part deforms and incapacitates in a short period, and it becomes necessary to replace the nozzle part frequently, which is obstacle to reducing production cost.
Thus, the conventional methods for manufacturing a granular silicon crystal have the problems of low productivity of a granular silicon crystal, wide variations in the particle diameter of manufactured granular silicon crystals, and high manufacturing cost.